Piezoelectric crystal device



Aug. 22, 1939. w. G. CADY PIEZOELECTRIC CRYSTAL DEVICE Filed April 27,1937 Patented Aug. 22, 1939 UNITED STATES PATENT Ot'Flif ApplicationApril 27,

9 Claims.

The present invention relates to piezo-electric crystal compressiondevices.

As explained in my Patents 1,450,246, issued April 3, 1923, and1,472,583, issued October 30,

1923, piezo-electric crystals have the property of becoming electricallypolarized when compressed or otherwise mechanically deformed, the

electric polarization giving rise to electric charges which can beutilized in various ways. A flat plate or rod cut from a crystal ofquartz or tourmaline in a direction perpendicular to an electric axis,for example, when compressed in the direction of its thickness, developsopposite electric charges on its oppositely disposed faces; and,conversely, when a voltage is applied to the said two faces by means ofsuitable electrodes, the crystal plate or rod becomes thicker orthinner, according to the direction. in which the voltage is applied.

The only crystals heretofore commonly used for this purpose have beenquartz and tourmaline. Prior to the present invention, it has beencommonly believed that crystals of Rochelle salt, which would be farmore useful, because they are many times more strongly piezoelectricthan either quarts or tourmaline, do not possess the very desirableproperty described above of becoming electrified in the direction of themechanical pressure, or vice versa. It has generally been supposed, upto now, that the piezo-electric nature of Rochelle salt is not such asto cause a plate of this subsance, when compressed, to take on oppositeelectric charges on its two faces.

Since the earliest investigations on piezo-elec- ,tricity in the lastcentury, it has been known that compressional piezoelectric effects areof two general types, known as longitudinal and transverse. These termshave reference to the relation between the direction of the appliedelectric 40 field and the resulting deformation of the crystal; or,conversely, the relation bet veen the direction of an applied force andthat of the resulting electric polarization. When a mechanical pressureis associated with a held in the same direction, the sheet is termedlongitudinal; when the field is at right angles to the pressure, it iscalled the transverse effect. Many piezoelectric crystals exhibit oneeiiect in one direction and another in some other direction, or botheffects 50 may be present for the same direction of field, as is thecase, for example, with quartz. The term crystallographic axes isemployed in this specification in the generally accepted sense, denotingan orthogonal system of axes based on the symmetry characteristics ofthe crystal. The

1937, Serial No. 139,227

term piezoelectric moduli, or its equivalent, denotes the modulireferred to this system of axes. Nine of the eighteen possiblepiezoelec' "ic moduli yield shears in various directions. In somecrystal classes, including that to which Rochelle salt belongs, the onlycompressional effeet, for an electric field parallel to one of thecrystallographic axes a, b and c of the crystal, is the transverseeffect. In the case of the Rochellesalt class, this may be expressed bythe statement that the only piezoelectric moduli are shear moduli,denoted by (Z14, (Z and the. In other words, the only type of strainthat is produced piezoelectrically by an, electric field in the crystalis a shear with respect to one or more of the three said orthogonal axesof the crystal. In my Patent 1,977,169 of October 16, 1934:, I havediscussed the nature of these moduli and pointed out some practicalapplications.

It has heretofore been considered, as before a stated, that in Rochellesalt and other crystals possessing only piezoelectric shear moduli, thelongitudinal effect does not exist; and this notwithstanding the accountwhich J. and P. Curie, the discoverers of piezoelectricity, give oftheir first experiment on Rochelle salt (Comptes Rendues, vol. 91, p.383, 1880) and notwithstanding the equations derived by Voigt, in hisLehrbuch der Kristallphysik, Leipzig, 1910, pp. 849 and 872.

Hitherto, all piezoelectric devices for producing electric effects frommechanical pressure, in which the longitudinal effect was employed, havemade use of plates cut at right angles to one of the crystallographicaxes. In this invention, 1 show that similar devices can be made usingplates cut from crystals in which there is no longitudinal effect withrespect to any single crystallographic axis. Even if there is alongitudinal effect with respect to one or more of the crystallographicaxes, it may be of advantage, for mechanical or electrical reasons, tocut plates according to the present invention in such direction withrespect to the crystallographic axes as to make possible a moreeffective use of the longitudinal effect.

It is therefore an object of the present invention to provide acompression piezoelectric device constituted of Rochelle salt or othercrystal of piezoelectric properties and exhibiting the longitudinalefi'ect.

Other and further objects will be explained hereinafter and will beparticularly pointed out in the appended claims.

The invention will now be described in connection with the accompanyingdrawing, in which Fig. 1 is a diagrammatic perspective view of aRochelle salt crystal, shown in broken lines, with the so-calledhemihedral (oblique) faces some what exaggerated, since, as is wellknown to those versed in crystal physics, these are the faces thatindicate the piezoelectric properties of the crystal, the said Fig. 1showing also the three crystallographic axes a, b and 0, together withan obliquely out plate which, for clearness, is drawn in full lines, inthe form of a triangle; Fig. 2 is a view of a plate cut in theorientation indicated in Fig. 1, with electrodes; and Fig. 3

is a view similar to Fig. 2 of a modification.

The flat triangular crystal plate is assumed, in Fig. 1, to be so cutfrom the mother crystal that its normal, which, for convenience, I callthe a-axis, makes equal angles with the three crystallographic axes a, band c of the crystal. The plate is shown in the form of a trianglemerely to make the principle clear; it may have any suitable shape, suchas rectangular. The dimensions of a suitable, rectangular plate 6 mayfor example, be approximately 3.5 mms. thick in the a direction, withparallel faces 35 mm. x 27 mm. Instead of a plate, a bar or rod I may beemployed, as in Fig. 3, say, approximately 19 mm. long in the adirection, with rectangular cross section 7.15 mm. x 5.70 mm., thelength of the bar or rod being disposed in the same direction beforedescribed.

It is not essential that the axis a make equal angles with thecrystallographic axes, but, for maximum piezoelectric effect, the threeangles should, in the case of Rochelle salt, theoretically be equal.This is not necessarily true, however, concerning crystals of classespossessing a longitudinal effect with respect to one or more of thecrystallographic axes, and which may also be within the presentinvention, as hereinafter described. For mechanical reasons, it may bedesirable to depart somewhat from this particular angle.

The longitudinal effect with the flat plate may be exhibited asillustrated in Fig. 2, in which the piezoelectric plate 6 is shownresting on a lower metal electrode '!,'with an upper metal electrode 8held by a stem 9 by means of which the position of the electrode 8 canbe regulated. If the electrode 8 is allowed to press with a known forceupon the crystal plate 6 and the electrode 7, then, when the electrodes7 and 8 are connected to a calibrated ballistic galvanometer, adeflection will be observed due to the charges liberated on theelectrodes i and 8 by the longitudinal effect. When the plate 6 iscompressed in the direction of its thickness, therefore, it also becomespolarized in the same direction, so that electric charges appear uponthe opposite faces. The converse is also true.

From this deflection the piezoelectric modulus, which I designate by dfor the longitudinal effect, can be calculated. In my experiments, thevalue d was found to be about 3 x 10 electrostatic units, which is insatisfactory agreement with the value expected from theory.

This value of the piezoelectric modulus d is about five hundred times asgreat as that of quartz or tourmaline. Herein lies the advantage in theuse of the longitudinal eifect of Rochelle salt. Though quartz andtourmaline are much stronger mechanically and less affected by pressure,temperature and moisture than Rochelle salt, nevertheless, fiat platesof Rochelle salt can be subjected without injury to Very considerablemechanical pressure, and their great superiority in the intensity of thepiezoelectric eifect compensates in large measure for their mechanicalinferiority.

The plate shown in Figs. 1 and 2, as also the rod of Fig. 3, may serveas a piezoelectric resonator, according to the principles set forth inmy said Patents 1,450,246 and 1,472,583. In order that the resonator mayvibrate freely, the upper electrode 8 is raised suificiently to leave asmall gap between it and the plate 8, as illustrated in Fig. 2; or, thecrystal plate may be coated on both sides with thin metal foil, forexample, gold foil, serving as electrodes. The electrodes 7 and 8 may beconnected to an electric oscillating .circuit of the right frequency, asillustrated in Fig. 3, and the presence of resonant vibrations may bedetected by the reaction upon the electric circuit. This may beindicated by an audible click, which may be heard in a telephonereceiver when the frequency passes through the resonant value; by asudden change in the reading of an arnmeter; or by the controllingeffect which the crystal exerts upon the frequency of the circuit. Theseare illustrated in the said patents and need not, therefore, beillustrated here. The observed resonant frequency with a plate 1.93 mm.thick was observed to be 1070 kilocycles per second, while the frequencyfor longitudinal thickness vibrations calculated from theory was 1060kilocycles per second. With plates of different thickness the frequencyis found to be inversely proportional to the thickness, which is furtherproof that the vibrations are in the direction of the thickness.

The rod I of Fig. 3 may be employed to demonstrate and confirm theexistence of the longitudinal effect. Though the apparatus indicated inFig. 3 is suitable for demonstration purposes, it is to be borne in mindthat, since the electric field is parallel to the length of the rod, themetal electrodes 2 and 3 must be located at the extreme ends, therebymaking the electric field comparatively weak. The electrodes 2 and 3,slightly separated from the ends of the red, are supplied withhigh-frequency alternating current from a suitable source 4. Under theaction of the longitudinal efiect, the rod becomes alternatelylengthened and shortened by the alternating electric field, resulting inlongitudinal vibrations which, at the resonant frequency, cause finemetallic particles sprinkled on the surface to be shaken off, exceptalong the nodal line 5, which, in a particular experiment, was found tobe disposed obliquely to the direction of the length of the rod. Theexistence of a node at the central portion of the rod demonstrates thereality of the longitudinal effect. The obhquity of the nodal lineindicates, as is well known to those versed in the art, that, while thelength of the rod was in the proper direction for maximum longitudinalexcitation, the direction of maximum elastic constant, along which thevibrations tend to take place, made a certain angle with the directionof the length. This is due to the well-known peculiar elastic propertiesof Rochelle salt. It is possible that more effective vibrations may besecured, even if at a slight sacrifice of piezoelectric activity, bycutting the rod or plate in a slightly different direction, so as tomake the nodal line at right angles to the direction of the length.

The fact that the rod is vibrating in resonance is further shown by itsreaction on the driving circuit, which reaction may be observed eitherby the sudden change in the reading of a meter, or by the click producedin a telephone receiver, as before described. The rod, in other words,through the action of the longitudinal effect, becomes a piezo-electricresonator, according to the principles described in my abovementionedPatents 1,450,246 and 1,472,583.

Just as with other well-known types of resonator depending upon thelongitudinal piezoelectric effect, so also in. the case of a plate outaccording to the present invention, it is possible, by application of avoltage of the proper frequency, to excite the plate so that it willvibrate at an overtone of its fundamental frequency. At the fundamentalfrequency the thickness of the plate is a half Wavelength of thecompressional Wave. \Vhen the plate vibrates at the first overtonefrequency, the thickness of the plate is approximately three halfwavelengths; or, in general, it is approximately equal to an odd numberof half wavelengths. Thus, a plate out according to the presentinvention may be used at overtone frequencies for the generation ofultrasonic waves in air or in any other gas, liquid or solid.

It follows. from the theoretical considerations mentioned above thatsuch a plate as that rep-- resented in Fig. 2, out according to thepresent invention, should function either as a micr phone or as areproducer for acoustic waves, whether sonic or ultrasonic. This I havefound experimentally to be the case. A Rochelle-salt plate of this typeWas provided with tinfoil electrodes and connected to the input of anampli her, the output of which was connected to a loud speaker. Whensound waves from the human voice or other sources fell upon the surfaceof the plate, it was found that the plate was set into vibration sothat, through the longitudinal piezoelectric effect, electric currentswere generated which caused the sound to be reproduced in the loudspeaker. A diaphragm having a central opening was placed in front of thecrystal in some of these tests, to make sure that the sound energy fellupon the surface of the plate and not upon the edges. ,4

It was also found that when the crystal plate was connected to theoutput of an amplifier, the input of which was provided with a currentof audio frequency, said plate was set into vibration and served as anemitter or rep-roducer cf sound over a very wide range of frequencies.As is usually the case with crystal reproducers and microphones, thisdevice was more effective at high than at low frequencies.

In the course of these reproducer tests a stethoscope was used toexplore the sound field close to the vibrating crystal plate. Owing tothe transverse effect, which theoretically is present along with thelongitudinal effect, some sound was emitted laterally from two oppositecorners of the plate, but most of it was given off uniformly from theflat tin-foil-coated face in a direction at right angles to the face.

The present invention thus provides a means for generating substantiallyplane waves of sound. especially sound of high frequency. This ispossible by the fact that the type of device here in described consistsof a flat plate, the entire ;major surface of which moves in and out inaccordance with electric impulses supplied to it, in contrast to othertypes of crystal sound-generators such as have been used hitherto, inwhich the moving portion of the crystal is relatively small. If a stilllarger vibrating area is desired than can be secured with a single plateof the type herein described, a plurality of plates can be assembledcovering a surface of any desired area, all connected to a common sourceof electric power.

In a similar manner, a single plate or a plurality of plates can be madeto serve as receivers of sonic or utrasonic waves by allowing such wavesto fall upon the surface of the plate or plates, the electrodes attachedto the plates being connected to any suitable amplifying and recordingor reproducing system.

It will be understood that the same technique and methods. of mountingheretofore in use in various piezoelectric crystal applications areequally applicable to the device of the present invention.

The invention is not, of course, restricted to Rochelle salt; it isapplicable to any piezoelectric crystal. A plate, bar, rod or the like,cut from any such crystal, in an orientation, oblique with respect tothe said axes, such as to become electrically polarized to substantiallythe maximum extent in the direction of its thickness when a mechanicalpressure is applied in that direction, is within the invention. Byhaving the device cut at an angle oblique to all the crystallographicaxes, the longitudinal effect will be scoured through the cooperation ofall the piezoelectric moduli that the crystal may possess. The inventionmay, as stated above, be employed with plates or other devices cut frompiezoelectric crystals of any class, even those possessing longitudinaleffects with respect to one or more of their crystallographic axeswhichis not true of Rochelle salt-provided that the plate or other device issuitably oriented with respect to the said axes. It may also be employedwith those crystal classes having no lon itudinal effect with respect toany crystallographic axis, and yet possessing moduli that are not shearpiezoelectric moduli. In many, if not most, cases where the crystalpossesses a longitudinal effect with. respect to one or more of itscrystallographic axes, the total longitudinal effect with an oblique cutcannot be expected to be materially greater than if the plate were cutin. the usual way, perpendicular to one of the crystallographic axes.Hence the invention applies more particularly to crystals possessingonly shear piezoelectric moduli. In all such cases, if the plate, bar,rod or the like is suitably oriented, it will become polarized in adirection having a component parallel to the direction of compression inthe thickness direction of the device and, conversely, will undergoextensional strain in the thickness direction when an electric field isapplied in this direction.

Though the invention is most useful when maximum effects are obtained,des ed above, it will be understood that its disting .ig fe tureconsists in cutting from the piezoelectric crystal a plate, rod, orother suitably shaped specimen, in such a manner as to be at obliqueangle to all of the crystallographic axes. By this means the variouspiezoelectric modu even in a crystal possessing no longitudinal 2 ectwith respect to any of its crystallographic may be caused to cooperatein such a way as to realize the above-mentioned longitudinal effect. Byproper choice of angular orientation, of course, this effect may be madeto assume a maximum value, as before described.

When the piezoelectric moduli of any crystal are known, the properdirection of out for maxi mum longitudinal efiect may be calculated fromwell known equations, such, for example, as given in the above-mentionedbook by Voigt, on pages 838 and 849. For crystals belonging to the cubichemihedral or tetartohedral classes, the tetragonal trapezohedralhemihedral and sphenoidal-hemihedral class, the rhombic hemihedralclasses (which includes Rochelle salt), and the hexagonalenantiomorphic-hemihedral class, the following equation holds:

wherein the polarization P, in the direction specified by the directioncosines Z, m and n, is expressed in terms of the pressure F in thisdirection, and the three piezoelectric moduli. Crystals of these sixclasses all have shear moduli, and only shear moduli, with respect tothe crystallographic axes. For convenience, they will be referred to inthe claims under the terminology the six piezoelectric shear classes, orits equivalent. The maximum value of P for these classes is obtained bymaking Z, m and n all equal, that is, by cutting the specimen so thatthe applied 25 force makes equal angles with the three crystallographicaxes. For most of the other piezoelectric crystal classes, the formulasare more complicated, but the direction for maximum longitudinal effectcan always be determined. It may, for example, be derived from thefollowing general formula (83) on page 849 of the abovementioned book byVoigt:

In this formula, the ds are the various piezoelectric moduli (some ofwhich are usually equal to zero for any particular crystal), and Z, mand 7 are, as before, the direction cosines. As is obvious to thoseversed in this field, the maximum value of P will in general not be suchthat Z, m and n are all equal, hence for such crystal classes themaximum polarization, while lying in a direction oblique to all threecrystallographic axes, will not make equal angles with these axes.Nevertheless, there is always a certain particular direction, specifiedby certain values, of Z, m and n, for which the polarization is amaximum.

It is well known that, in the past, for certain special purposes, plateshave been out, making an oblique angle with one or more of thecrystallographic axes. However, heretofore no such cuts have been madefor the purpos eof obtaining a maximum longitudinal effect. It is truethat, in general, every oblique cut may be expected accidentally tocontain a trace of the longitudinal effect, to some slight extent. Ingeneral, however, the effect will be small unless the angle of cut isproperly chosen, as above described, Within certain limits.

The invention has many uses, such as in acoustics, the generation ofsupersonic waves, underwater signalling, in piezoelectric devices fortesting or measuring various mechanical effects, such as pressures andvibrations in machinery and explosives, and the control or measurementof highfrequency electric currents.

Other modifications will occur to persons skilled in the art, and allsuch are considered to fall within the spirit and scope of theinvention, as defined in the appended claims.

What is claimed is:

1. A device cut from a piezoelectric crystal of the rhombic-sphenoidalclass, the device being cut at substantially equal angles to thecrystallographic axes of the crystal.

2. A Rochelle-salt crystal device cut at substantially equal angles tothe crystallographic axes of the crystal.

3. A Rochelle-salt crystal resonator cut at substantially equal anglesto the crystallographic axes of the crystal and provided with electrodesdisposed substantially perpendicular to the normal to the resonator, theresonator having a nodal plane disposed substantially at right angles tothe direction of the length of the resonator.

4. A piezoelectric device for producing electric effects from mechanicalpressures, consisting of a plate cut from a Rochelle-salt crystal in adirection making substantially equal angles with all of thecrystallographic axes, provided with suitable electrodes and employingthe longitudinal effect.

5. A piezoelectric generator of sonic or ultrasonic waves, consisting ofone or more plates cut from a piezoelectric crystal belonging to one ofthe classes that do not posses the longitudinal piezoelectric elfectwith respect to any one of the crystallographic axes, the plate beingcut from the piezo electric crystal in a direction making substantiallyequal angles with all the crystallographic axes and capable of being setinto thickness vibration through the longitudinal piezo-' electriceffect when excited by an alternating current of sonic or ultrasonicfrequency.

6. A piezoelectric generator of sonic or ultrasonic Waves, consisting ofone or more plates, provided with suitable electrodes cut from aRochelle-salt crystal in a direction making substantially equal angleswith all the crystallographic axes and capable of being set intothickness vibration through the longitudinal piezoelectric efiect whenexcited by an alternating current of sonic or ultrasonic frequency.

'7. An acoustic device for emitting or receiving sound waves, consistingof a plate cut from a piezoelectric crystal belonging to one of theclasses that do not possess the longitudinal piezoelectric effect withrespect to any one of the crystallographic axes, the plate being cutfrom the piezoelectric crystal in a direction making substantially equalangles with all the crystallographic axes. so as to exhibit tosubstantially a maximum degree the longitudinal effect.

8. An acoustic device for emitting or receiving sound waves, consistingof a plate cut from a Rochelle-saltcrystal in a'direction making substantially equal angles with all the crystallographic axes, providedwith suitable electrodes and employing the longitudinal effect.

9. A piezoelectric device comprising a plate cut from a piezoelectriccrystal belonging to one of the classes that do not possess thelongitudinal piezoelectric effect with respect to any one of thecrystallographic axes, the plate being cut from the piezoelectriccrystal in a direction making substantially equal angles with all thecrystallographic axes so as to exhibit to substantially a a maximumdegree the longitudinal piezoelectric effect.

WALTER G. CADY.

